Even though House Republicans put national security at the top of their 2016 agenda, it may still be difficult for GOP lawmakers to pass a new measure authorizing military force against the Islamic State.

The House has already begun informal talks on the legislation, known as an Authorization for Use of Military Force, or AUMF, following the spread of the terrorist group in the Middle East and deadly attacks in Paris and San Bernardino, Calif.

But lawmakers aren't sure what to include in a formal measure that would likely increase U.S. military presence in the fight.

The last time Congress voted on such a measure was in 2002 when lawmakers authorized the war with Iraq under President George Bush. Many lawmakers currently serving in the House weren't elected yet when the vote took place.

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